1. Field of the Invention
The invention is directed to an improved fuel injection system for internal combustion engines.
2. Description of the Prior Art
A fuel injection system of the type with which this invention is concerned is known, for example, from the document DE 197 01 879 A1 and includes a fuel tank from which fuel is delivered into a high-pressure accumulation chamber by a high-pressure pump. A control unit maintains a predetermined high fuel pressure in the high-pressure accumulation chamber. A number of high-pressure lines, which corresponds to the number of combustion chambers of the internal combustion engine, each lead from the high-pressure accumulation chamber to a respective fuel injection valve; the fuel injection valve can be connected to the high-pressure line by means of a control valve. The control valve and the fuel injection valve here are frequently disposed in a single housing to save space. The fuel injection valve here has a valve needle, which is guided in a bore and is encompassed by a pressure chamber in the region oriented toward the combustion chamber. A pressure surface is embodied on the valve needle, which is acted on by the fuel in the pressure chamber so that when a particular opening pressure is achieved in the pressure chamber, the valve needle executes a longitudinal movement counter to a closing force, thus unblocking at least one injection opening through which fuel travels from the pressure chamber into the combustion chamber of the engine. The control valve of the fuel injection system is embodied as a 3/2-port directional-control valve, which in one position, connects the high-pressure accumulation chamber to the pressure chamber of the fuel injection valve and in a second position, breaks the connection to high-pressure accumulation chamber and connects the pressure chamber to an overflow fuel chamber, which is provided in the valve body and which is connected to the fuel tank by means of a line, so that a low fuel pressure constantly prevails in the overflow fuel chamber. If the control valve switches from the closed position into the open position, then a pressure wave is produced, which travels through the supply conduit into the pressure chamber and produces an overpressure there, i.e. the injection of the fuel occurs at a pressure that is considerably higher than the pressure in the high-pressure accumulation chamber. As a result, high injection pressures are achieved with a moderately high pressure in the high-pressure accumulation chamber and in the high-pressure fuel-carrying parts of the fuel injection system. Since the fuel in the supply lines is in motion because of the open control valve during the injection, when the control valve is closed, the moving fuel is abruptly stopped so that the kinetic energy of the fuel is converted into compression work. This produces pressure oscillations, which complicate the precise metering and proportioning of the injection quantity in a second injection immediately following the first injection, since the state of the control valve is not precisely known due to the pressure oscillations.
The object of the current invention, therefore, is to design a fuel injection system, which permits a more precise metering of the injection quantity and more precise setting of main injections, preinjections, and secondary injections.